Plate-shaped quenching elements have been known for a long time from the prior art. These are generally made from iron sheet and combined to form a quenching sheet package or a quenching unit. A quenching sheet package typically consists of around 5 to 20 quenching elements. In technical circles such plate-shaped quenching elements are also referred to as quenching sheets.
The quenching function of the quenching unit on the one hand requires the electrical and also the magnetic conductivity of the iron. Due to the magnetic conductivity an arc occurring when the main contacts of the switching device are opened is drawn into the quenching unit and quenched there. In other words the arc is moved away from the switching contacts to the quenching unit by the magnetic suction effect. When the arc enters the electrically conducting quenching sheets, it is divided into individual segments and new arc base points form on the quenching sheets. These have a significantly higher voltage requirement (typically 10-20 V) than the actual arc plasma, so that a current-limiting effect is initiated, as a result of which the arc quenches. Some of the quenching sheet material is also evaporated. The evaporation energy is drawn from the arc. A higher pressure also results in the quenching unit due to the vapor. Both effects bring about a greater arc voltage requirement for the arc and therefore improved current limiting or accelerated quenching of the arc. The voltage requirement necessary to quench the arc is typically approximately 1.5 to 2.5 times the mains voltage to be shut down by the switching device.
The disadvantage of this variant is that the current to be shut off in the event of a short circuit is relatively poorly limited due to the high electrical conductance of the iron or of ferromagnetic metals. The exiting metal vapor to some extent produces an improvement in the electrical conductivity of the arc plasma, which on the one hand disadvantageously reduces the voltage requirement of the plasma. On the other hand the residual conductivity of the plasma cannot be reduced sufficiently quickly as a result during the current zero and some instances of so-called re-ignitions result after the initial quenching of the arc, in other words the current circuit is as if closed again or later shut off.
A further disadvantage of ferrous quenching sheets is that they rust without surface treatment and this can result in a loss of function (either due to the coalescence of the quenching sheets due to rust bubbles or due to the electrical insulation of the rust). As a result quenching sheets have to be galvanically surface-refined for example. However this surface protection evaporates with the first short-circuit shut off so the problem occurs “anew” for further operation.
Such switching devices are correspondingly voluminous and complex in order to be able to cope with the high current strengths. The short-circuit current can be a multiple of a rated current to be switched operationally, such as 10 times up to 1000 times for example.
The use of plastics that emit large quantities of gas in the switching chamber is known from the prior art for short-circuit current limiting. To this end the plastics are provided with a flame retardant, e.g. aluminum hydroxide or magnesium hydroxide. In the event of arc contact the plastic decomposes and emits gas. The energy required for endothermic decomposition of the plastic is drawn from the plasma of the arc in this process. The release of gas results in a higher mass density or higher pressure in the plasma, with the result that heat can be drawn from the arc plasma more readily. The voltage requirement of the arc increases. The emitted decomposition gases thus have a significant current-limiting effect.
Because the plastic parts are in contact with the flame retardant in the switching contact region, decomposition of the plastic parts also takes place during operational switching. These disadvantageously emit gas prematurely, so that in some circumstances the current-limiting effect of the gas-emitting plastics is no longer ensured in the event of a short circuit.
The decomposition products can also be deposited on the switching contacts of the switching device. This disadvantageously results in an increase in the contact resistance and thus to an erroneous increase in the heating of the switching device.
So-called current-limiting polymer compounds, which are utilized in switching devices or are additionally connected in series, are also known from the prior art. Such a compound or such a polymer current-limiter can be connected directly in the current path or in the quenching circuit of the switching device.
The polymer current limiter has the task of increasing the switching capacity of the switching device used. In this process it remains in rated operation, in overload operation and in the cased of smaller short circuits inactive. Only in the case of larger short-circuit currents does the polymer compound intervene with a sudden resistance increase to limit the short-circuit current. The switching device therefore does not have to be designed for the maximum possible short-circuit current, just for the short-circuit current limited by the polymer compound.
One disadvantage of this is the additional space requirement for the incorporation of the polymer compound in the switching device.
It is also disadvantageous that the polymer compound is connected electrically in series and also produces an additional electrical power loss due to its path resistance even in rated or overload operation. Also with such current limiters that have to be activated thermally incorrect switching can result (early tripping), electrical loads such as motors are started frequently at short time intervals. The starting current flowing on starting here corresponds to approximately 6 to 10 times the rated current, so the polymer compound is heated significantly.
Such a polymer compound consists of an electrically non-conducting phase, the so-called matrix, and a conductive phase, in other words a filler material. Generally a plastic, in particular a thermoplastic, is used as the matrix and carbon black as the filler material. Metal particles or graphite can be used as an alternative to carbon black.
The three-dimensional arrangement of the filler material particles in a two-component system, consisting of the matrix and the filler material, is also referred to as a percolation network. Depending on the shape and distribution of the filler material particles there is a certain filler material concentration, from which a closed path of filler material particles results through the two-component system. The filler material concentration is also referred to as the percolation threshold. In other words a specific electrical conductivity value for the polymer compound can be predetermined by appropriate selection of the filler material concentration.